How to understand type of variable speed systems for lathes.

Well folks I hope I don't bore you, I'll try and make his simple to understand. Just a little about me. I have a degree in Physics and have over 30 years as an applications engineer applying variable speed drives to industrial machines. I have expierience with everything from small 1/10th HP all the way up to 10,000 HP.
First a fiew principles to understand.
HP is the ability to do work over time. The definition of HP is 1 HP is the ability to lift 33000 lbs 1 foot in 1 minute. When applied to rotary motion then HP = (torque x RPM)/k. The K is 63025 if we are working with in.lbs torque and 5252 if we are working with ft.lbs. So working backwards 1 HP x63025/1750RPMs would mean we would get 36 in.lbs of torque. 1 hp x875/63025=72 in. lbs. That is exactly how a step pulley or a reeves drive works. As the speed goes down then the torque goes up accordingly and vice a vesa.

Part II

Now variable speed motor controls are a little differant.
for small vgariable speed drive that are redily availabe for home shop use the are 2 kinds. 1st there is the DC Variable speed control. This one works with either a perminate magnet motor or a wound field motor. The way it works is we vary the voltage to the armature of the mtor to vary the speed. The vary design of the DC motor is such that it will produce a constant torque at any speed. Don't ask me how because it would require another thread and many pages to explain.
For an AC Motor the speed of the motor is determined by the number of rotating magnetic poles it has and the bapplied frequency. Thats why some will refer to a 3600 (3450) RPM motor and a 1800(1750) RPM motor as a 4 pole motor. Thats 3000 RPM and 1500 RPMS for our 50 cycle friends. what a VFD does is generate a variable frequency output. The key to the vfd is that if maintains a constant volt/cycle when it varys the frequency it also varies the output voltage. This is done to keep from burning oiut the windings. on a 230 volt control we would see 3.8 volts per cycle.
A lower voltage will result in a lower torque and a higher voltage will burn out the windings. You can do something wonderful on a VFD that make it a good choice for lathes. That is you can also make it into a constant Horsepower device. The way this is done is by lowering the voltage while increasing the frequency. Lets say we have a standard 4 pole motor and we want to apply it to a lathe the way I would do it is I would use a set of pullies to gear it so that the lowest speed would match the motor base speed that is I'd gear it so the 1750 RPM Motor would be turning my lathe the owest desired speed. The I'd simply operate my VFD in a mode where I hold the voltage at a constant 230 volots and nincrease to frequency now when I am operating the motor at 120 Cycles I'd be going 3450 RPM. Now the drive is acting like a reeves drive giving me the variable speed constant Horsepower.
Unfortuatly this is not a practle way to apply the system since this is ony a 2:1 speed range. The more practicle way would be to gear the base speed to the lathe so that when we are a base speed we are at the middle speed for the lathe. Now I can operate the motor in a constant torque for half the speed range and constant HP for the upperhalf. I I don't have enough Horsepower at the lower speed than I would simply select a larger HP mtor say a 2 HP Motos instead of a 1 Horsepower. How I'll have 1 HP at 875 RPMs and 2HP from 1740 to 3450 RPM's
I hope that I have mode this clear as mud . Any questions

Huh?

This is a little easier for me to understand.

1. Always set the speed control knob to its lowest setting before starting lathe.
2. Flip toggle switch to ON position to start motor.
3. After lathe has started, turn knob clockwise to increase spindle speed.

Chuck Rodekohr said:

This is a little easier for me to understand.

1. Always set the speed control knob to its lowest setting before starting lathe.
2. Flip toggle switch to ON position to start motor.
3. After lathe has started, turn knob clockwise to increase spindle speed.

Click to expand...

You obviously have a Shopsmith.

It should get easier when they come out with the Power Pro Headstock this summer with a variable speed, reversing, higher HP motor.

http://www.shopsmith.net/forums/announcement.htm?f=6

Paul Gallian said:

http://en.wikipedia.org/wiki/Horsepower

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Paul if you would like I can take the basic definition of horsepower and translate into any of the forms that you have cited but for the purposes of this discussion I have tried to simplyfy things. Let me know if I can clarify things for you since I am only trying to help a few folks understand some of the basics of physics.

slapping myself for replying!

Joking aside, it is mostly understandable and I thank you for the trouble of posting. It will be around for a long time so others can use for reference.
If there is an issue, I believe most of the explanation has little practical application for the average lathe user.
It works or doesn't. Pretty basic.
I would add one thing to what some others have posted. Stand to the side the first time you hit the on switch with a hunk of wood mounted. That way you can watch the flying object instead of feeling it hit yer face.

Don,

Thanks for the explanation. I am a mechanical engineer so I understand the HP, torque stuff. The sparky stuff is where I am a little fuzzy.

This helped clear things up for me.